Haloalkanes — Part 2: Substitution, elimination, metals and polyhalogen compounds: visual lesson, ISC Class 12 Chemistry

Lesson

Reactions of haloalkanes

Part 2 — substitution, elimination, reactions with metals, and some important polyhalogen compounds.

In this lesson you will

Draw the SN1 and SN2 mechanisms and explain their stereochemical outcomes
Write the products of haloalkanes with common nucleophiles, including KCN/AgCN and KNO₂/AgNO₂
Predict the major alkene using Saytzeff’s rule; write Wurtz and Grignard equations
Describe chloroform, iodoform and other polyhalogen compounds and their environmental effects

Concept

Substitution by nucleophiles

A nucleophile replaces the halide: $\ce{Nu- + R-X -> R-Nu + X-}$ . Ambident nucleophiles (CN⁻, NO₂⁻) attack through C/O with ionic K salts but through N with covalent Ag salts.

Table of substitution products of haloalkanes with nucleophiles
Reagent	Product from R–X
Aqueous NaOH or KOH	Alcohol, R–OH
Moist Ag₂O (or boiling water)	Alcohol
NaI in acetone	Iodoalkane, R–I
NH₃ (excess, alcoholic)	1° amine, R–NH₂ (further alkylation possible)
R′NH₂	2° amine, R–NHR′
KCN	Nitrile (alkyl cyanide), R–CN
AgCN	Isocyanide, R–NC
KNO₂	Alkyl nitrite, R–O–N=O
AgNO₂	Nitroalkane, R–NO₂
R′COOAg	Ester, R′COOR
LiAlH₄	Alkane, R–H

Concept

The SN2 mechanism

One step: the nucleophile attacks from the side opposite the halogen as the C–X bond breaks. Rate $= k[\ce{RX}][\ce{Nu-}]$ (second order). Configuration inverted. Reactivity: CH₃X > 1° > 2° > 3° (steric hindrance).

Table of SN2 reactivity
Halide	SN2 rate	Reason
Methyl	Fastest	No crowding
Primary	Fast	Little crowding
Secondary	Slow	More crowding
Tertiary	Very slow	Backside blocked

Cookies